docs: def assert_in(first, second, msg_fmt="{msg}"):
      """Fail if first is not in collection second.
  
      >>> assert_in("foo", [4, "foo", {}])
      >>> assert_in("bar", [4, "foo", {}])
      Traceback (most recent call last):
          ...
      AssertionError: 'bar' not in [4, 'foo', {}]
  
      The following msg_fmt arguments are supported:
      * msg - the default error message
      * first - the element looked for
      * second - the container looked in
      """
  
      if first not in second:
          msg = "{!r} not in {!r}".format(first, second)
          fail(msg_fmt.format(msg=msg, first=first, second=second))